Can you book a reservation at age 18 for one of the cabins or do you have to be 21?

You should be able to rent a room at 18, but you will need some kind of card to reserve it, and for incidentals. The cabin owners may have their own policy, but generally at 18 you are allowed, unless there is a mini bar in the room. Which, in this case there is not. I'd call the owners.

I do not know the answer, but you can contact them online.